The Fairbanks Morse model FMJ magneto
was used extensively by AC for the model B tractor. My 1942 B has
always had one. Here's what it looks like:

You'll notice the metal thumb tab
that is used to ground the magneto and stop the engine. My B was a
wartime B and was not originally supplied with electrics. There was
no switch/ammeter panel up by the seat. In order to stop the engine,
one had to climb down off the tractor and walk around it in order to
depress this tab and stop the engine. I have since added electrics
and wired this terminal back to a switch on the switch panel. It's a
lot safer that way.

The B flywheel has two different
timing marks on its' circumference. One mark is "FIRE" and
the other is "CENTER". These marks are visible at the
correct times through the timing inspection opening on the left side
of the tractor and the forward end of the torque tube where it
mounts to the engine block flange. It is an opening about 1 inch in
diameter and is shown here:

The FIRE mark is visible in the
opening when the number one piston is 30 degrees before top dead
center on the compression stroke. The CENTER mark is visible when
that same piston is at top dead center (TDC), also on the
compression stroke. If you ever have the engine removed, that would
be an excellent time to clean up and clearly identify these two
markings, usually with some paint. Either of these marks can be used
to time the magneto to the engine but each one uses a different
method. If you just wanted to check the timing with the engine
running using a timing light, you'd attach the timing light pickup
to the number one cylinder sparkplug wire and use the FIRE mark. If
you just wanted to check the timing without the engine running, you
would use the hand crank, slowly rotate the engine in the normal
direction (CW looking to the rear). When the number one piston comes
up to TDC on compression, the magneto impulse should just snap and
the CENTER mark should be visible in the inspection opening. You
have to go slow to do this properly.

If you have removed the magneto main
cover to check or adjust the points (0.020" is the correct
setting when on a lobe high point), when reassembling the cover
you'll need to make sure the drive gear and the pinion gear are
properly meshed. Here's how to do that:

The magneto for the B is set up
clockwise rotation so you'll want to use the "C" mark on
the fiber drive gear. It may be difficult to see. Try cleaning the
gear with a dry cloth. Do not use a solvent or you may obliterate
any remaining evidence of that mark. You may even need a magnifying
glass to find it. Once found, mark that valley between two teeth
well for the next time. The mating steel driven gear has one tooth
that is "dog-eared" or chamfered to mark it. I have read
that some mags may have a spot of red paint to mark such a tooth. It
can be a little difficult to make sure these marks line up when
replacing the main cover since you can't see them as the cover goes
down. I usually just line up the "C" mark by rotating the
rotor so that the "C" is at the very bottom. I then
rotate the steel gear using the mag drive tabs at the impulse
coupling and rotating backwards (CCW looking at the impulse) such
that the marked tooth is at the very top. Without
moving either gear I then carefully lower the main cover down onto
the mag body until the teeth mesh. Once this is complete, you can
then begin to time the magneto to the engine. If, for some reason
you cannot locate the "C" mark on the fiber gear or the
chamfered tooth on the steel gear, a recommended timing procedure is
given at the end of this write-up.

As I indicated before,
there are two different ways to time the magneto to the engine, depending on whether you
want to use the FIRE mark or the CENTER mark on the flywheel. The
two methods are described here:

The "Advanced Spark Position
Method" described above uses the FIRE mark. The "Impulse
Coupling Trip Method" uses the CENTER mark. I personally prefer
the Impulse Coupling Trip Method, so that is the one I will explain.
Either method will work fine if done properly.

The first thing to do after you are
sure the mag is assembled properly itself is to set up the engine
such that the number one piston is at top dead center (TDC) on the
compression stroke. The cylinders are numbered one to four starting
at the front as shown here:

I take all the spark plugs out to
make the engine easy to turn over slowly. You need to rotate the
engine slowly until the number one piston begins the upward motion
on the compression stroke. To do this I usually just put my thumb
over the number one sparkplug hole while rotating the engine with
the crank. Once I begin feeling air pressure building up under my
thumb, I know that I am on the compression and not the exhaust
stroke. It helps if two people are available at this point but it
can be done alone. Rotate the engine slowly in the normal direction
(CW looking toward the rear). You will need to keep a close eye on
the inspection opening at the flywheel end at the same time. Make
sure you rotate slowly. You will first come upon the FIRE mark when
the crank is 30 degrees before TDC. Continue rotating slowly until
the CENTER mark is visible in the opening and then stop. At this
time the two magneto drive tang recesses should be approximately
horizontal. If they are not, you have an internal engine timing
issue that needs to be corrected before continuing.

The next step is to set up the
magneto properly to mate with drive tang recesses in the engine
noted above. You need to get the mag in a position where the number
one cylinder contact has just fired using the impluse coupling. The
cylinder contact towers in the mag outer cover are indicated here:

I use a sparkplug cable inserted in
the number one tower and a sparkplug to determine when fire occurs
at that tower as shown here:

Just lay the plug on the metallic
portion of the mag so as to ground it. The impulse coupling will
rotate two complete revolutions for each revolution of the rotor.
The impulse coupling will snap twice per revolution of the coupling.
The spring is fairly stiff so I usually use a pair of vise grips on
one of the two drive tangs to get some leverage. Don't touch the
sparkplug or the cable or you might get a good shock! I usually
steady the mag by pressing on the main cover and slowly
rotating the impulse coupling (CW when looking at it) with the vise
grips. When the impulse coupling snaps (trips) at number one
cylinder you should see a spark at the sparkplug. If you have gone
though four snaps and have not observed a spark, make sure you have
a good ground where the plug contacts the mag body. You may want to
use a clip wire from the plug body to the mag body to insure that
ground. You need to rotate slowly to be sure that when it snaps and you see the
spark, you do not continue to rotate past the trip point. You want to
stop immediately after the trip. At this point, the drive tangs at
the impulse coupling should be almost horizontal as shown here:

You should now be able to easily
couple the magneto to the engine without having to rotate either.
The drive tangs on the impulse coupling should mate right up with
the corresponding recesses in the engine block. The timing is now
complete. Just install all plugs, hook up the sparkplug cables as
shown in the pictures above (firing order is 1-2-4-3) and the engine
should fire right up.

If you want to use the other
"Advance Spark Position Method", set the engine up the
same way, but stop at the FIRE mark with number one cylinder on the
compression stroke. Set up the mag by removing the outer cover and
inspecting the rotor contact position with respect to the timing
boss as shown in the method above. Make sure you rotate the mag in
the direction opposite normal - CCW when looking at
the impulse coupling - when you line up the rotor contact with the
timing stud. You will not be able to line it up by rotating CW
because the impluse coupling windup will stop the rotor contact
before the stud and then snap it rapidly well past it. The mag
should now couple right up to the engine as with the other method.
You may find this way to be faster and easier. I just prefer the
other one because I dislike removing either of the two mag covers
unless I absolutely have to. The sealing on these covers is very
important to keep moisture out of the mag. Because my B is outside
year-round that sealing is especially important to me.

For additional information on the FMJ
magneto, see a cross-sectional view and parts list available here.

Note: I have never actually performed
this procedure myself but believe it will work fine for those who
are unable to locate the magneto internal timing marks.

1. Set up the tractor with the "Fire" mark on the flywheel centered in the timing inspection opening when
#1 piston is coming up on the compression stroke. The "Fire" mark is located 30 crank degrees before
the "Center" mark at top dead center (TDC).

2. With both the large main and small rotor covers off the magneto, turn the impulse coupling backwards
(CCW when looking at it) as needed, such that the coupling drive
tangs mate up with the recesses in the
governor drive when the magneto body is in the middle of its' timing adjustment range.

3. Leaving the small rotor cover off, rotate the rotor (and fiber gear) in the large main cover until the rotor
contact lines up with the timing lug.

4. Carefully assemble the main cover to the magneto body without further movement of the rotor. A very
minor adjustment may be necessary to get the gears to mesh - but it should be minimal.

5. Install the small rotor cover, attach the plug wires in the appropriate firing order and start the engine.

6. Timing can be checked using a timing light attached to the #1 cylinder ignition cable with the engine
running. With this method, the "Fire" mark should be visible in the timing inspection opening during
timing light strobes. It can also be checked statically by turning the engine over slowly by hand. With this
method, the impulse coupling should snap and a spark occur at the #1
plug when that piston is at TDC on the compression stroke and the "Center" mark is visible in the timing inspection opening. In
either of these methods of checking the ignition timing, a minor adjustment can be made, if necessary,
by loosening the magneto attachment bolts and rotating the magneto body within the adjustment range. Rotating the top of the magneto out away from the engine block advances the spark. Rotating it in
towards the block retards it.